Generally speaking, once-through evaporator technology may be employed within generating systems such as, for example, steam generating systems, and include multiple heat exchange sections or stages. Typically, there are two heat exchange stages. In a first or primary evaporator stage, a fluid such as, for example, feed water, is partially evaporated to produce a steam/water mixture. In a second or secondary evaporator stage the fluid is further evaporated to dryness and the steam is superheated.
As shown in FIG. 1, a conventional once-through evaporator 10 includes heat exchange stages, e.g., primary evaporator stage 20 and secondary evaporator stage 30 that each includes a parallel array of heat transfer tubes 22 and 32, respectively. Mass flow rate within internal portions of the tubes 22 and 32 is controlled by buoyancy forces, for example, density differences induced by heat transfer to the fluid in the tubes such that the mass flow rate is proportional to the heat input to each individual tube within the arrays of tubes 22 and 32. One type of evaporator uses vertical tubes arranged as a sequential array of individual tube bundles. Each tube bundle (e.g., a bundle 32A of FIG. 1), also referred to as a harp, has one or more rows of tubes that are transverse to a flow of a hot gas 40 (e.g., a flue gas). The individual harps 32A are arranged in the direction of gas flow so that a downstream harp (e.g., a harp 32B) absorbs heat from the gas of a lower temperature than the upstream harp 32A. In this way, the heat absorbed by each harp in the direction of gas flow is less than the heat absorbed by the upstream harp.
As shown in FIG. 1, the primary evaporator stage 20 (e.g., the array of tubes 22) receives a fluid 12 (e.g., feed water) at an inlet manifold 24 and distributes a water/steam mixture 14 (e.g., a two-phase flow) from an outlet manifold 26 of the primary evaporator stage 22 into the secondary evaporator stage 30 (e.g., the array of tubes 32) where dry-out and superheating takes place. The secondary evaporator stage 30 includes a plurality of inlets 34, one or more inlets at each of the harp bundles of the secondary stage 30. As such, the two-phase flow 14 passes through each branch of the secondary stage 30, e.g., harps 32A and 32B, and the harps disposed therebetween.
Operating experience has shown that flow instabilities can develop in the primary evaporator stage 20, which can lead to fluctuating temperatures within the tubes 32 of the secondary evaporator stage 30. The fluctuating temperatures can lead to fluctuating thermal stress within the tubes and may result in various tube failures such as, for example, tube cracks. Techniques are known to minimize flow instabilities in the primary evaporator stage. For example, it is known that by increasing the pressure drop across individual harps within the array of tubes 22, flow rates that would normally be controlled by buoyancy can be overcome. Techniques employed include installing an orifice in the inlet of each row of the tubes 22 or reducing an inside diameter of the inlets or tubes themselves.
Calculations show that different distributions of resistance for each row of tubes in the primary evaporator maintain stability over a range of operating conditions. However, this limits the stable operational range for a given primary evaporator configuration. For example, a set of orifices designed to provide stability at full load operation may not be effective in partial load operation. As such, instabilities may occur during operation at partial loads. Moreover, an additional problem that can limit the operation of the evaporator at low load is that at low mass flow rates the velocities in the downcomer, e.g., conduit 28 of FIG. 1, that passes the two-phase flow 14 from the outlet manifold 26 of the primary evaporator stage 20 into the secondary evaporator stage 30, may become too low to carry steam bubbles down and away from the outlet manifold 26. As a result there can be a build-up of steam either or both in a top portion of the downcomer (conduit 28) and/or at the primary evaporator outlet manifold 26. A build-up of steam may induce additional flow instabilities.
Accordingly, there is a need to develop systems and methods for mitigating flow instabilities and fluctuating thermal stress that can result therefrom to minimize tube failure.